The invention is related generally to power supply systems and more particularly, to downhole instrument power supply systems in which shunt voltage regulation is used.
Downhole camera systems permit the visual inspection of the interiors of otherwise inaccessible underground areas such as well casings. Such camera systems typically include a downhole "head" having a camera, a lighting system for illuminating the area being viewed by the camera, and other electronic components which perform control and data transfer functions. The head is connected to a surface power source and processing system by means of an umbilical cable.
In downhole camera systems, voltage regulation is usually needed because of the voltage sensitivity of one or more devices in the downhole head. Although some components in the downhole head, for example certain cameras, may operate acceptably on a relatively wide voltage range, for example, anywhere from 9 to 18 volts direct current (VDC), other equipment, such as data communications circuits, require voltage within a relatively narrow range, for example 12 VDC.+-.5%.
Downhole video camera heads typically use 120 VDC/100 watt halogen lamps for illumination. The illumination provided by such lamps is directly related to the voltage provided to the lamp. A variable DC power supply is located at the surface to provide power for the lamp, camera and associated downhole electronics. The surface power supply is variable to allow the operator to adjust the lamp intensity for various hole conditions. A typical range is 40 through 120 volts at the downhole lamp. The camera and associated electronics require 12 VDC and must take that power from the same power source as the lamp. Therefore, a voltage regulator included in the downhole instrument is often used. As an example, a voltage regulator having the following specifications may be used:
output voltage of 12 VDC.+-.0.1%; PA1 output current of 250 milliamperes (ma) average; PA1 input voltage variations from 40 through 120 VDC; PA1 operating temperature range from 0.degree. to 100.degree. C.; and PA1 maximum power dissipation of less than 10 watts.
Surface control over the intensity of the downhole illumination has been found to be desirable due to the variability and unpredictability of conditions which may be found downhole. In some cases, immiscible media exist in a well, for example, water and oil. It has been found that it would be of value to visibly detect such media for the purpose of locating the entry point in the well of one or more of these media. For example, in an oil well having one or more side branches, it would be of value to lower the camera head to the points of these side branches to see if water is entering the well at those branches. Corrective action could then be taken if water is detected. It has been noted that such media detection is made easier when illumination at decreased levels is used. The layers of the media can more easily be seen under these reduced illumination levels whereas they are more difficult to see under high illumination levels.
On the other hand, inspection of a well casing having a turbid medium contained within requires a high intensity illumination so that the illumination will pierce the turbidity and illuminate the well casing. Illumination at low intensity would, in this case, be of limited value. Thus illumination control is desirable.
Due to size limitations and levels of heat to which a downhole camera head can be exposed during use, it is desirable to keep the downhole circuitry at a minimum in size, complexity, heat sensitivity, and heat generation, yet, voltage regulation is nevertheless required. Camera heads for well holes must be rugged to withstand the sometimes harsh conditions encountered in typical operation. For example, hydrostatic well pressures in excess of 4.2.times.10.sup.6 kilograms per square meter (6,000 pounds per square inch) and high ambient well temperatures are not uncommon. This high heat in the environment makes it desirable to reduce the amount of internal heat generated by the camera head itself.
Because of the long lengths of umbilical cable often used with its inherent resistance, the voltage reaching the downhole camera head may vary from the voltage required, for example 12 VDC. In some cases, the voltage source at the surface is increased slowly from its minimum voltage, such as 40 volts, to a level above the downhole selected level. The voltage regulator in the camera head maintains the voltage applied to the electronics at the selected level regardless of how high above that level the voltage reaching the camera head from the surface is. In such a voltage regulator system, the surface operator need only set the surface voltage at some level above the selected level, and the downhole regulator will operate to reduce the voltage arriving through the umbilical cable to the desired level. For example, the surface voltage may be set at 40 VDC, the voltage arriving at the camera head may be 18 VDC due to cable resistance and the voltage regulator will reduce that voltage to 12 VDC before applying it to the camera head electronics.
It would be desirable for the technique used to regulate the voltage at the camera head to also contribute to controlling the illumination in response to surface action. This would result in fewer components in the camera head.
A further consideration in providing a power supply system having a downhole voltage regulator is the possibility of creating undesirable electromagnetic interference. If such interference is severe enough, signals may be corrupted. In many cases today, analog type cameras are used in the downhole camera head. The analog signal from the television camera, traveling on a lengthy cable to the surface, is particularly vulnerable to electromagnetic noise generated by any downhole device.
In the past, some downhole video camera systems have employed switching type voltage regulators. The 120 volt maximum input voltage of the voltage regulators in some systems coupled with the wide input voltage range (40 to 120 VDC in some systems) drove many designers to use switching technology. However, switching regulators have drawbacks that are unavoidable. They are noisy and complex. Electrical noise in a camera system is not desirable because it can degrade picture quality. Complexity in any downhole instrument, especially one used in the oil service industry, reduces reliability and makes repair difficult. The 100.degree. C. temperature requirement further complicates the design.
Additionally, many switching type voltage regulators generate enough electromagnetic noise to corrupt the video signal sent to the surface. In many cases complex circuitry must be designed to remove this switching noise from the video display. This custom designed circuitry is not only costly but it also adds another element to the system which is subject to failure.
Driven by the desire for noise free, very simple, and highly reliable regulators, various analog regulators have been considered. The classic analog voltage regulator is a series pass element type. This type is very simple and reliable; however, because of the high input voltage, the pass element, such as a transistor, would have to dissipate an unacceptably high amount of power. The amount is unacceptable because it limits the regulator's use to environments having lower maximum temperatures. Above that maximum environmental temperature, the heat built up in the instrument would be above the transistor's operating temperature.
Hence, those skilled in the art have recognized the need for a downhole instrument power supply system which allows for surface control over the power applied to a downhole energy transducing device, such as a lamp, for controlling the output of that device while delivering a relatively precise regulated voltage to other co-located devices. Those skilled in the art have also recognized the need for a power supply system which has a reduced potential for generating electromagnetic noise which may interfere with the operating of other system components. There also exists a need for a design which generates less internal heat. The present invention fulfills these needs and others.